A four-cycle engine mounted upon a motorcycle is usually of the overhead cam type and is provided with a cam chamber within which a cam chain for driving the cam is accommodated and which is usually integrally formed within and with a cylinder block and a cylinder head.
Lubrication for a variable valve mechanism, a large diametered portion of a control rod of the engine cylinder, a bearing portion of the crank shaft, and the like of the engine of the type described above is performed by means of lubricating oil pumped from an oil pan disposed within a lower bottom portion of a crank case by means of an oil pump through an oil passage formed so as to penetrate a thickened portion of the crank case and an oil passage composed of a recessed groove formed within a mating face of the crank case to be mated with a side cover.
Lubrication inclusive of the cooling of the piston is further performed for achieving the lubrication between the cylinder and the piston and between a small diametered portion of the control rod and a piston pin by properly supplying oil to the lower surface of the piston by suitable oil jetting means.
In accordance with conventional technology, the oil jetting means is, in one arrangement, disposed directly below the lower end of the piston when the same is positioned at a lower dead position and is embedded within the upper end portion of the side wall of the crank case so that the front end of the jetting means is directed toward the inner surface of the piston through means of a gap defined between the lower end opening of the cylinder and the outer peripheral side surface of the crank web. The oil is supplied to an oil passage formed so as to penetrate a thickened portion of the side wall of the crank case.
In accordance with another arrangement, the oil jetting means is itself disposed at the same position as described with respect to the above arrangement, but in this arrangement, the oil is supplied to the jetting means through means of an oil passage formed within the outer periphery of a bearing of the crank shaft.
In both of these arrangements, however, since the oil jetting means are embedded within oblique holes or bores formed within the upper end portion of the side wall of the crank case, a drilling working operation is required in order to form the inclined hole, which requires a drilling operation which is different and independent from the other operations, resulting in an increase of the usage of additional tools, of the manufacturing processes and, hence, of the manufacturing cost. Furthermore, in a certain case, it may be troublesome to form the oil passage within the lower portion of the crank case so as to avoid any interference with the location of the other members or elements disposed upon the side surface of the crank case.
In accordance with another method for forming the oil passage as a passageway, a recessed groove within the mating faces of a side cover mounted upon the side surface of the crank case, for example, a clutch cover, and the crank case. In accordance with this structure, an oil connection passage is provided for the oil jetting means so as to thereby simplify the entire structure.
However, in the case where the cam chain chamber is disposed between the clutch cover and the side wall of the crank case, it is difficult to provide the oil passages so as to bypass these portions, and thus the simplification of the structure of the oil passages even in accordance with the method described above cannot readily be realized.